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How to introduce a pleco to a goldfish tank 
Ask how large the pleco will grow to be. 
Although most pet stores will sell plecos that are only a few inches in length, you should know that they can grow quite large. Fully grown plecos can reach up to two feet in length.
The exact size of the pleco will vary depending on the species. Ask someone at the pet store how large the pleco will get before making a purchase.